Electrophoretic study of the small subunits of the myosin molecule

Editorial: Subunits of myosin. Relations to ATPase activity and mechanical function of muscle

Under certain conditions the specific ATPase activity of myosin of a given muscle can be altered. The cause of this alteration can only lie in the myosin molecule itself. To produce an enzymatic activity of myosin, an interaction between their light and heavy chains is necessary. However, the specific activity appears to be determined mainly by light chains. Hence, one ought also to look for a basis of the changed activity in changes of the subunits of myosin. There are strong indications that the alterations in specific activity are accompanied by changes in the relative stoichiometry of the essential light chains of the respective myosin preparation. They differ in their pattern of subunits. The specific activity of a given kind of myosin seems to be determined by the combination of their light chains. Thus, a close correlation exists between these two properties of myosin (ATPase activity and structure of its molecule). There are sufficient indications, that these two properties of myosin correlate also with the mechanical capabiltiy of the corresponding muscle. Particularly the results of cross innervation studies demonstrate a close correlation between these three properties in skeletal muscle. The single subunits of myosin are produced and degraded independently and at heterogenous rates. The synthetis of these subunits is significantly accelerated in response to work overload. Thus, it is quite likely that the individual chains are non-coordinately synthesized, giving rise to variations in the relationship of different molecule types of myosin with different specific ATPase activity. Hence, the control mechanism to synthesize the individual subunits could also be the regulative mechanism to produce a myosin of the specific ATPase activity appropriate to the activity pattern of tissue.

Identification of the essential light chains of myosin

The molecular weights of light chains associated with adult and embryonic chick myosin have been determined by polyacrylamide gel electrophoresis in the presence of 0.1% sodium dodecyl sulfate. Adult muscle myosin contains three light chains with molecular weights averaging 27,700, 21,000, and 16,500, while the embryonic form contains only the two largest of these three. Recombination and hybridization experiments have been performed with these samples. The data clearly demonstrate that only two light chains are required for the expression of the full ATPase activity of myosin. The third light chain consistently is associated with adult myosin, but definitive evidence for its role is lacking.